Optical fiber communications have developed into ring-type structures and are further developing into mesh-type structures by the wavelength division multiplexing (WDM) technique utilizing the characteristics of optical fiber communications, having started from a system that connects two points with large capacity. In a photonic network having optical fibers connected in a mesh-like manner, each node having optical fibers connected thereto preferably has the function to direct signals to different destinations.
After an optical signal is converted to an electric signal, the electric signal is converted back to an optical signal based on the destination to which the electric signal is directed, and the optical signal is transmitted to the next node. This is a stable technique, but gives rise to enlargements of device size, power consumption, and the like. In view of the above, there has been provided an optical ADM (Add/Drop Multiplexer) that can switch wavelength paths by diverging or inserting optical signals of certain wavelength (see “Optical Add/Drop Multiplexer in Metro/Access Network System” by Masayoshi Kagawa, et al., Journal of the Furukawa Electric Co., Ltd. January 2003).
A ROADM (Reconfigurable Optical Add/Drop Multiplexer) that can diverge or insert optical signals of desired wavelength has the advantages that optical paths can be arbitrarily set by remote control, and a photonic network that does not depend on bit rates can be constructed. With such a ROADM, a network operation becomes easier, and more flexible construction of photonic networks is realized.
However, a ROADM does not have the function to input and output optical signals of the same wavelength at high speed in a time domain, though being flexible in inputting and outputting wavelengths of optical signals. To realize a photonic network with higher granularity in the future, it is preferable to develop an optical switching technique by which optical signals can be arbitrarily input and output at high speed in a time domain. Therefore, intensive studies are being made on switches for inputting and outputting optical signals at high speed in a time domain.
A technique for switching optical signal paths in a time domain is disclosed in “All-Optical 80-Gb/s Add-Drop Multiplexer Using Fiber-Based Nonlinear Optical Loop Mirror” (by Ju Han Lee, et al., IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 17, NO. 4, April 2005 (hereinafter referred to as Reference 2)). According to this technique, with the use of an optical switch using an interferometer called NOLM (Nonlinear loop mirror) formed with optical fibers, optical signal paths are switched on the time axis by an interference action using control light. The NOLM is a high-speed and wide-band device, using a nonlinear effect of the optical fibers. Unlike regular optical fibers for transmitting optical signals, nonlinear fibers having a high nonlinear effect can form a small-sized structure. Also, electricity is not applied in the NOLM.
The technique disclosed in Reference 2 can be clearly distinguished from techniques using electronic circuits. However, it is difficult to maintain stable operations of the interferometer switch using the interference of light in the NOLM, and it is also difficult to increase the switching efficiency due to insertion loss of an optical component or the like. As a result, the optical signal quality is degraded. In a future photonic network that is expected to have a number of nodes for an optical signal to pass through, it is preferable to achieve higher switching efficiency.